According to the bank's analysis, commodity price inflation in the past few months would justify a 3pc-3.5pc increase in processed food prices, but supermarkets have increased prices by 6pc-6.5pc.

Paul Donovan, a UBS economist who co-wrote the report, said: "That suggests there may be margin expansion in the supermarket sector… Prices are rising in excess of justifiable cost increases."

He added that such profiteering by supermarkets was adding 0.3 to 0.5 percentage points to the official consumer prices index of inflation, currently 4pc. The research also showed shoppers were seeing steeper rises in food prices than any other member of the Organisation for Economic Co-operation and Development (OECD).

According to UBS, food prices in Britain are rising at an annual rate of 4.9pc, compared with 3.6pc in Germany, a eurozone average of 1.8pc and a US increase of 1.5pc.

The weak pound has contributed to the higher-than-average UK increase, as 18pc of food consumed in the UK is imported, but the scale of inflation has made Britain the OECD country "most vulnerable" to political interference, Mr Donovan added. The report stated: "UK food prices are rising more rapidly than most other OECD economies' food prices, and have significantly outstripped food retailers' cost inflation.

This could allow UK politicians to suggest food price inflation is 'unfair' or 'excessive'."

It said supermarkets may come under political and media pressure to cut prices, or even face another competition inquiry.

Rising food prices do not have such a large impact in developed countries as emerging markets because most of the final supermarket price consists of labour, packaging, marketing and distribution.

Only 20pc-25pc of the final price reflects the "commodity input", UBS said. Even such a commoditised product as milk, for example, retails at 59p-79p a litre, when the farm gate price is just 26p a litre.

A British Retail Consortium spokesman said: "There is no question that in the UK, customers pay less for their food than is the case in most other European countries. Food prices have not risen at anything like the same rate as commodity prices. It is clear that supermarkets are shielding customers from the full impact."